By means of dilution developing process for the silver halide holographic plate, the influences of exposure energy on refractive index and index modulation in emulsion were experimentally researched. According to the exposure characteristics of optical density and diffraction efficiency, the absorption modulation of silver-halide emulsion with dilution developing process will be ignored. The hologram is a volume phase grating with an average absorption coefficient but no absorption modulation. The index modulation mechanism of the gratings was discussed, and the characteristics curves of refractive index and index modulation of effective medium in silver-halide emulsion were revealed. The maximum of index modulation of silver halide emulsion can be reached at equally intensity between reference and object waves. The test results in carlier papers have been made clear.